I keep gaining weight, but I don't eat much, is there a medical reason for this?

Possibly. Hypothyroidism (low thyroid hormone), cushing's syndrome (too much cortisol), med side effects, inactivity, fluid retention; there are many reasons why weight could increase despite not eating much. Regardless, apart from retaining too much fluid (eg. Congestive heart failure), if weight is increasing, it is because the number of calories exceeds the body's need.
Maybe. There r some medical reasons for weight gain (water retention with chf, severe hypothyroidism or cushings). Altho hard to believe, many ppl eat a lot more than they realize bc processed foods and sauces sneak in a lot of calories and portions have been distorted by restaurants.